The armies of Yazid bin Muawiyya were all Muslims, during Salat they would send benedictions upon the Prophet Mohammad and His family, yet they didn’t care for upholding respect and reverence for the Prophet and His family and they killed Imam Hussain (AS) and his companions in Karbala.
The question is what was wrong with their Islam and its practice? What was missing from their Islam? Did they practise Islam perfectly?
Alas! The Muslims do not want to know what caused those Muslims to kill the family of the Prophet!
I want to tell you the fact that their practice of Islam was imperfect, and they lacked the knowledge of the heart, thus, they failed to absorb the divine light in their hearts. The Kalima (declaration of faith) they recited would not go beyond their throats. Their hearts were null and void of Emaan (faith) and Noor (divine energy).
O Muslim, if you have the same deficiency today, you are also vulnerable to becoming yet another Yazid.